Small gas turbine engines, such as those used as auxiliary power units ("APU"), are normally controlled by metering the output of a fixed displacement fuel pump that is driven at a constant fraction of engine speed through a geared transmission. The metered flow rate is typically controlled through an engine speed control loop to achieve the desired engine power setting. In such systems, excess fuel pump output is typically bypassed across a relief valve, and constitutes a significant inefficiency.
During engine start up, it is necessary to control the mass flow rate of fuel from the engine-driven pump without the benefit of an engine speed control loop, since prior to engine light-off, fuel delivery rate has no influence on engine speed. This typically requires ancillary elements, such as a pressure regulator, a metering valve and some type of flow-measuring instrument. All of these elements add significant cost to the engine control system. Moreover, mass flow rate is a function of temperature, a variable that can quickly change. For example, a commercial jetliner may quickly move from one location, at which the tarmac temperature may exceed, say, 100.degree. F., to another location at which the ambient temperature is less, say, than 32.degree. F. It is common to operate the APU while the aircraft is being serviced at the gate. However, the mass flow rate of fuel necessary to start the APU will have to be adjusted for differences in the ambient temperature.
Accordingly, it would be generally desirable to provide an improved method and apparatus for starting a gas turbine engine that would omit the need for these elements. By omitting these, the overall cost of the engine control system can be reduced. Additionally, if such a simplified system were also to permit fuel pump speed to be controlled independently from that of the engine, then improved power efficiency during normal engine operation would also be realized.